I'd like to see more first-era cultures.  Later eras let you transcend, so you effectively have more than ten options (even if only ten options are fully optimal gameplay-wise), but the first era is more limited.  You're also pretty limited geographically.  Here are some I was thinking of to add a bit of geographic diversity:

Chavín - Builder (South America)

Đông Sơn - Scientist (Southeast Asia)

Jomon - Agrarian (Japan)

Lapita - Expansionist-Naval (Oceania)

Nok - Aesthete (West Africa)

Poverty Point (is there a better name?) - Builder (North America) 

Scythian - Militarist (Eurasia)

Únětice - Merchant (Central Europe)

Urewe - Agrarian (Subsaharan Africa)

A fantasy mod that replaces the animals in the game with dinosaurs. It'd be a world similar to old caveman fantasy movies, Frank Frazetta paintings, or Genndy Tartakovsky's Primal. That would be fun for me.

A tunnel district is really needed in the game right now. 

There should be a mod that displays the user interface as well as the management of all important resources and status information without having to search for them.

Another thought of mine would be whether there could not be a mod that at least the strengths of the avatar for the user to unlock or make usable, that could give the game more fun in my opinion.